从函数返回指向全局数组的指针

Question

I am not experienced enough in C/C++ programming, so I am asking for an explanation.

I have global array declared as following. ASAK it is located in seperate memory part of initialized global memory in context of process memory.

Sensor sensorsArray[SENSORS_COUNT] = {dhtTempSensor, dhtHumSensor, dallasTempSensor, waterLevelSensor};

I need to find element in this array and return pointer to it (because I am going to change its value). I have written such function.

Sensor* getSensorById(uint32_t id) {
  for (int i = 0; i < SENSORS_COUNT; i++) {
    Sensor* current = &sensorsArray[i];
    if (current->sensorId == id) {
      return current;
    }
  }
}

Will it work properly, I am not sure about current pointer, it is allocated on the stack so it is in function scope, will it be poped from the stack after function ends ? Or it will work properly.

I mean not pointer(address of array element which is taken using &sensorsArray[i]), but current pointer variable which contains address of erray element, will it be poped or not.

Please suggest best way how to do in such situation. Thx.

Answer 1

You aren't covering all the possible returning cases of the function, namely, the case when the id does not match with any of the id s of the array.

Currently the pointer will return the last element of the array if there is no match.

You could correct that by defining the pointer Sensor* sensor_found = nullptr outside the for loop such that if there is no sensor found the return value is still valid, ie nullptr and assigning the found value of current to sensor_found , only if there is a match.

Sensor* getSensorById(uint32_t id) {

    Sensor* sensor_found = nullptr;

    for (int i = 0; i < SENSORS_COUNT; i++) {

        Sensor* current = &sensorsArray[i]; 

        if (current->sensorId == id) {
             sensor_found = current;  
             break;    
        }
    }

    return sensor_found;
}

If the id found return current , otherwise, if there is no match return nullptr .

Answer 2

you want to make sure that the function has a valid return statement on its every execution path. In you current implementation if the id is not matched then the return value of Sensor* is not set and will contain random bytes. One wau to deal with this situation is to return the nullptr to indicate that the Sensor was not found. Other than that, ythe function will work properly.

Sensor* getSensorById(uint32_t id) {
  for (int i = 0; i < SENSORS_COUNT; i++) {
    Sensor* current = &sensorsArray[i];
    if (current->sensorId == id) {
      return current;
    }
  }
  return nullptr;  // id not matched
}

Answer 3

Your code is fine (as the comments suggest). The reason why you don't need to worry about the current pointer becoming invalid is because the memory that it points to (ie, the global array) stays valid beyond the scope of the function. Just because you happen to create a pointer (and remember, a pointer is really just a number that corresponds to some place in memory) to that memory doesn't mean that it becomes invalid when used elsewhere.

When you say Sensor *current = &sensorArray[i]; , then if sensorArray[i] is stored at, say, position 0x10 in memory, then current = 0x10 , and no matter where it is used, then sensorArray[i] will still be at memory location 0x10 . When you assign a value to current , you are not copying the value from the sensor, you are merely getting a pointer to it.